1. Field of the Invention
The present invention relates to an optical pick-up device for recording information on a magneto-optic disc and reading the information recorded, and more particularly to an optical pick-up device capable of not only accurately detecting focus errors and tracking errors, but also accurately detecting magneto-optic signals and pit signals recorded on an optical disc by use of a dichotomous hologram without using any diffraction lattice.
2. Description of the Prior Art
Referring to FIG. 1, there is illustrated a conventional optical pick-up device. As shown in FIG. 1, the optical pick-up device includes a laser unit 1 for emitting a laser beam, a diffraction lattice 2 for diffracting the laser beam emitted from the laser unit 1, thereby generating main beams and sub-beams for error detection, and a collimator lens 3 for converting the beams emerging from the diffraction lattice 2 into parallel beams. A polarized-beam splitter 4 is disposed downstream of the collimator lens 3. The polarized-beam splitter 4 serves to reflect all of S-polarized beams emerging from the collimator lens 3 and a part of P-polarized beams, namely, beams polarized in perpendicular to the S-polarized beams. The polarized-beam splitter 4 also transmits the remaining part of the P-polarized beams therethrough. The optical pick-up device further includes a reflection mirror 5 for perpendicularly reflecting the beams linearly polarized in the form of P-type wave by the polarized-beam splitter 4, and an object lens 6 for focusing the parallel beams of P-type wave reflected by the reflection mirror 5 onto an optical disc 11. A modified wollaston prism (MWP) 7 is also provided. The MWP 7 serves to convert the beam reflected by the optical disc 11 into a parallel beam through the object lens 6 depending on the presence of information recorded on the optical disc 11. The MWP 7 also separates the beam including three kinds of beams, namely, all of S-wave, a part of P-wave and a mixed beam of S-wave and P-wave, into five kinds of beams at different angles through the reflection mirror 5 and the polarized-beam splitter 4. The optical pick-up device further includes an imaging lens 8 for receiving the five kinds of beams separated by the MWP 7 and generating an image from the received beams, a concave lens 9 having a toric surface adapted to increase the divergence angle of the beams emerging from the focusing lens 8 and generate an astigmatism of the main beams (P-wave+S-wave) for detection of focus error, and a photo detector 10 for receiving the beams emerging from the concave lens 9 and detecting focus error, tracking error and optical information from the received beams.
In the conventional optical pick-up device having the above-mentioned arrangement, P-wave and S-wave have a certain relation.
Assuming that an X-axis extends in a direction perpendicular to the travel direction of a laser beam, the P-wave corresponds to a beam polarized in a direction perpendicular to both the X-axis and the travel direction of the laser beam. On the other hand, the S-wave corresponds to a beam parallel to the X-axis. Typically, the laser beam emitted from the laser unit 1 is at a polarized state.
The laser beam emitted from the laser unit 1 is subjected to a diffraction by the diffraction lattice 2, thereby generating three kinds of beams, namely, main beams and two kinds of sub-beams. The three kinds of diffracted beams enter the collimator lens 3 and then converted into parallel beams which are, in turn, incident on the polarized-beam splitter 4.
The polarized-beam splitter 4 reflects totally the S-wave components of the main beams emerging from the collimator lens 3 in the form of parallel beams at the boundary surface 4a thereof while transmitting the P-wave components of the main beams and the sub-beams therethrough. The P-waves and the sub-beams transmitted through the polarized-beam splitter 4 are fed to the reflection mirror 5 and then perpendicularly reflected by the reflection mirror 5.
The P-polarized beams and the sub-beams emerging from the reflection mirror 5 are concentrated by the objection lens 6 so that they are focused onto a track T1 of the optical disc 11.
The main beams M1 focused onto the optical disc 11 are used for reading of information and detection of focus error whereas the two kinds of sub-beams SB1 and SB2 are used for detection of tracking error.
The main beam M1 of P-wave and two kinds of sub-beams SB1 and SB2 are then reflected from the optical disc 11 while carrying information (pit information or information elliptically polarized by magnetization) recorded on the optical disc 11. Thereafter, these beams are incident on the object lens 6 which, in turn, converts the received beams into parallel beams again. The parallel beams are fed to the reflection mirror 5 and then reflected by the reflection mirror 5. The beams from the reflection mirror 5 enter the polarized-beam splitter 4. Where the optical disc 11 has information recorded thereon, an elliptical polarization occurs, thereby generating beams of S-wave.
The beams of S.-wave are mixed with the beams of P-wave to compose the main beams M1. Together with the two kinds of sub-beams SB1 and SB2, the beams of S-wave are incident on the polarized-beam splitter 4. Where the optical disc 11 has no information recorded thereon, however, only the beams of P-wave and the two kinds of sub-beams enter the polarized-beam splitter 4.
The following description will be made in conjunction with the case wherein the optical disc 11 has information recorded thereon.
All the S-wave components of the main beams M1, a part of the P-wave components of the main beams M1 and two kinds of sub-beams SB1 and SB2 incident on the polarized-beam splitter 4 are reflected at the boundary surface 4a of the polarized-beam splitter 4 and then fed to the MWP 7.
The two kinds of sub-beams SB1 and SB2 incident on the MWP 7 pass directly through the MWP 7 to be incident on the imaging lens 8. On the other hand, the main beams M1 are splitted into three kinds of beams while passing through the MWP 7.
In other words, the main beams M1 are splitted into beams of S-wave, beams of P-wave and beams of a mixture of P-wave and S-wave. Accordingly, 5 kinds of beams totally including the three kinds of beams and the two kinds of sub-beams are incident on the imaging lens 8 to generate an image.
The five kinds of beams emerging from the imaging lens 8 are then fed to the concave lens 9 having the toric surface, thereby increasing the divergence angle thereof. By the concave lens 9, an astigmatism occurs at the main beams for the detection of focus error. The five kinds of beams emerging from the concave lens 9 are then focused onto the photo detector 10 which has octant regions, as shown in FIG. 6.
The octant photo detector 10 detects a tracking error from a signal difference between the two kinds of sub-beams respectively focused onto the first and second regions 10a and 10b by the concave lens 9.
In other words, assuming that Sa and Sb are the signal indicative of the sub-beams focused onto the first region 10a and the signal indicative of the sub-beams focused onto the second region 10b, respectively, the tracking error signal TES can be derived from the following equation (1): EQU TES=Sa-Sb (1)
On the other hand, the focus error can be detected from the signal difference of the main beams focused onto the fifth to eighth regions 10f to 10h of the photo detector 10 caused due to the astigmatism as the distance between the optical disc 11 and the object lens 6 varies.
In other words, assuming that Se, Sf, Sg and Sh are the signal indicative of the main beams focused onto the fifth region 10e, the signal indicative of the main beams focused onto the sixth region 10f, the signal indicative of the main beams focused onto the seventh region 10g, and the signal indicative of the main beams focused onto the eighth region 10h, respectively, the focus error signal FES can be derived from the following equation (2): EQU FES=(Sf+Sg)-(Se+Sh) (2)
Where no focus error occurs, the main beams are focused onto the fifth to eighth regions 10e to 10h of the photo detector 10 in the form of a circular polarized beam, as shown in FIG. 3A. Where a focus error occurs, the main beams are focused in the form of an elliptical polarized beams, as shown in FIGS. 3B and 3C.
According, where neither of the tracking error and the focus error, both the tracking error signal TES and the focus error signal FES become zero (TES=0, FES=0).
Where the information recorded on the optical disc 11 is a magneto-optic signal (Kerr rotation based on magnetization), it is detected from the signal difference between the P-wave or S-wave focused onto the third region 10c and the P-wave or S-wave focused onto the fourth region 10d.
In other words, assuming that Sc and Sd are the signal indicative of the beams of S-wave focused onto the third region 10c and the signal indicative of the beams of P-wave focused onto the fourth region 10d (of course, the P-wave and S-wave may be incident on the third and fourth regions, respectively), the optical information signal can be derived from the following equation (3): EQU Optical information signal (magneto-optic signal)=Sc-Sd (3)
Where the information recorded on the optical disc 11 is a saw-shaped pit signal, the optical information signal can be derived from the following equation (4): EQU Optical information signal (pit signal)=Sc+Sd (4)
However, the conventional optical pick-up device for magneto-optic discs should use the diffraction lattice for detecting tracking error in accordance with the three-beam process. The optical pick-up device should also use the concave lens with the toric surface involving a difficulty in manufacture and an expensive cost and the modified wollaston prism involving a difficulty in manufacture for detecting focus error in accordance with the astigmatism process and reading information recorded on a magneto-optic disc. Furthermore, the conventional optical pick-up device should employ an optical system requiring an increased number of constituting elements. As a result, the pick-up device is increased in its weight, thereby resulting in a degradation in reading rate and an increase in manufacture cost.